1. Field of the Invention
The present invention relates generally to an optical reproducing head and, more particularly, is directed to an optical reproducing head of the type using a semiconductor laser source.
2. Description of Prior Art
There has been proposed an optical reproducing head according to the prior art using a semiconductor laser source which will now be described with reference to FIG. 1.
In FIG. 1, the semiconductor laser source 1 is formed of a semiconductor of the double hetero junction type, for example, of GaAlAs (gallium-aluminum arsenide). Laser source 1 is adapted to emit a laser beam having a cross-sectional shape at its emitting position of an elongated rectangle with approximate dimensions of 0.5 .mu.m.times.5 to 10 .mu.m. The laser beam emitted from laser source 1 is a diverging beam having anisotropic diverging angles and which forms an angle of about 30.degree. at the long side of the above cross-sectional shape with respect to the peripheral surface of a beam parallel to each long side, and about 4.degree. at the short side thereof with respect to the peripheral surface of a beam parallel to each short side, respectively. However, respective diverging angles of opposite peripheral surfaces of the beam are symmetrically arranged with respect to each other.
The diverging beam (linearly polarized laser beam) from laser source 1 falls on collimator lens 2 where it is converted into a beam of substantially parallel rays of light (a substantially plane wave beam) before falling on a cylindrical lens 3 (actually consisting of two lenses). At lens 3, the anisotropy of the beam converging angles is corrected and the collimated beam strikes a polarized beam splitter 4. Then, the linearly polarized laser beam from beam splitter 4 impinges on a .lambda./4 plate 5 where it is then converted into a circularly polarized laser beam, which then falls on an objective lens 6. The beam passed through lens 6 is converted into a focused beam having a substantially circular cross-sectional shape, and is thus finally focused on an optical record medium 7 so as to form a spot having a diameter of substantially one .mu.m or less.
On optical record medium 7, a pulse-code-modulated (PCM) information signal, such as a video signal, an audio signal or the like, is recorded as a train of bits on a track of a spiral form. A reflected beam from record medium 7 passes through objective lens 6 to strike .lambda./4 plate 5 and is thereby converted from a circularly polarized beam back to a linearly polarized beam (that is, the incident beam and polarization surface of .lambda./4 plate 5 meet each other at right angles). The converted beam from .lambda./4 plate 5 is reflected by beam splitter 4 to strike a cylindrical lens 8 and then falls on a photo-detector 9 (such as a PIN diode) to derive therefrom a reproduced output.
The photo-detector 9 consists of four rectangular photo-detecting elements which have the same dimensions. A spot of the incident beam supplied to photo-detector 9 is varied in shape from an ellipse through a true or perfect circle to another ellipse (whose long diameter intersects at right angles with the long diameter of the former ellipse) by operation of cylindrical lens 8 in accordance with the focusing state of incident light supplied to optical record medium 7. By utilizing the difference between the sum of reproduced signals at two opposing photo-detecting elements arranged on one diagonal line of detector 9 and the sum of reproduced signals at two opposing photo-detecting elements arranged on the other diagonal line, a focus error signal can be provided. This focus error signal is adapted to control apparatus for moving objective lens 6, or the whole optical system, in the direction of the optical axis of light impinging on record medium 7 so that the focus servo operation can be performed.
By utilizing the difference between the sum of reproduced signals at two photo-detecting elements of the right side and the sum of reproduced signals at those of the left side, a tracking error signal can also be provided. This tracking error signal is adapted to control apparatus for moving objective lens 6, or the whole optical system, in a direction perpendicular to the track on the record medium so that the center position of the beam can be controlled to accurately move on or follow the track.
In place of cylindrical lens 8, a wedge can be provided to separate the beam emitted from beam splitter 4 into two parts and the separated beams are then irradiated to photo-detector 9. In this case, the above separating angle is varied according to the focusing state of the incident beam on optical record medium 7, so that the two parts may be utilized to provide the focus error signal from photo-detector 9 in the same manner as described above.
When a semiconductor which produces anisotropic diverging angles which are small is employed as semiconductor laser source 1, cylindrical lens 3 can be omitted. However, such a laser source will seldom be used since its laser beam output is not sufficiently large.
However, the prior art optical reproducing head described above has many drawbacks. In particular, since the lenses used for collimator lens 2 and objective lens 6 are similar to the objective lenses of a microscope, the lenses are relatively heavy, for example, weighing as much as 8 to 10 grams. In addition, the optical system is of a relatively large size as a whole since the number of components used is large and therefore, the optical system occupies a large space. The fabrication and adjustment of the optical system are also troublesome, in that various components of the system begin to warp with the passage of time resulting in further cost increases. Further, when the optical system is moved up and down in a focus servo mode or vibrated right and left (wobbling) in a tracking servo mode, the driving device used for moving the system in the above modes is large. This results in the power consumption therefor being increased and the upper limit of the frequency range in the above vibration mode being lowered.
Another prior art optical reproducing head using a semiconductor laser source will now be described with reference to FIG. 2, in which elements corresponding to those described above with reference to the system of FIG. 1 are identified by the same reference numerals and with a detailed description thereof being omitted. In the system of FIG. 2, photo-detector 9 is arranged at one side of collimator lens 2 with semiconductor laser source 1 being interposed therebetween. A reproduced or reflected beam from record medium 7 is transmitted back through the path of objective lens 6, cylindrical lens 3, collimator lens 2, to semiconductor laser source 1, wherein the strength of the oscillating beam thereof is modulated according to the level of the returned beam. Thus, the strength of the beam radiated back from laser source 1 is detected and a corresponding signal is reproduced by photo-detector 9.
It should be appreciated that the optical reproducing head of FIG. 2 is of a slightly simpler construction in comparison with the optical reproducing head of FIG. 1. However, in order to perform respective servo actions by the focus and tracking error signals, the optical system must be subjected to a wobbling motion, and driving means for such motion is required. This results in a rather complicated construction, and whereby the stability in the servo mode is not very good.
The Applicants herein have proposed another optical reproducing head using a semiconductor laser source in which a hologram lens is used in the optical system, as shown in FIG. 3. With this optical reproducing head, a hologram lens 10 is provided in place of collimator lens 2, cylindrical lens 3 (which is sometimes not used), and objective lens 6 used in the example of FIG. 2, to achieve the latter's optical functions. When hologram lens 10 is irradiated with a laser beam from semiconductor laser source 1 which is similar to that of FIG. 2, a focused beam identical to that from objective lens 6 of FIG. 2 is obtained from the hologram lens 10. The remaining operation is also similar to the embodiment of FIG. 2.
Referring to FIG. 4, a description will now be given of a method of producing hologram lens 10. In FIG. 4, a hologram record medium 10' of hologram lens 10 is provided and a photo-sensitive layer (not shown) with gelatin as its base material is coated on a glass plate of record medium 10'. A photographic interference image or pattern, as will be described later, is formed on the photo-sensitive layer, and the treated photo-sensitive layer is developed to produce hologram lens 10.
In particular, a hologram lens 11 (in this case, an off-axis hologram lens) is provided as an objective lens for producing hologram lens 10. In other words, off-axis hologram lens 11 is disposed parallel to and opposite to record medium 10' with a predetermined distance therebetween. In front of hologram lens 11 is disposed a mask 12 which consists of a light transmitting portion 12a provided at its center and a light intercepting portion 12b occupying the remaining area other than the former portion, as shown in FIG. 5. For example, a metal coating such as chrome or nickel is selectively formed by deposition on a glass base plate to produce mask 12. The light transmitting portion 12a is made substantially coincident with the amplitude transmission factor of the emitting portion of a laser beam from the semiconductor laser source to be used, that is, the dimensions thereof are made substantially coincident with the cross-sectional configuration (and dimensions, of course) of the above emitting portion. In this example, light transmitting portion 12a corresponds to the shape of the laser beam from semiconductor laser source 1 previously described in FIG. 1.
A laser beam from a common laser source (not shown) is partially passed through a beam splitter 13 and falls on a beam expander 14 where the beam has its width expanded. In this case, it is preferable that the common laser source generates a laser beam having the same wavelength as the laser beam from the semiconductor laser source to be used. The expanded beam from beam expander 14 is reflected by a mirror 15 and the reflected beam therefrom falls on the off-axis hologram lens 11 at an incident angle of, for example, 45.degree., as a reproduction reference wave beam 16 (a plane wave beam or similar spherical wave beam). A focused subject wave beam 17 (a spherical wave beam), which is focused to a point P is thus produced by hologram lens 11 in response to reference wave beam 16. The subject wave beam 17 is irradiated onto record medium 10' in front of point P as a focused record beam (a spherical wave beam) with its optical axis perpendicular to record medium 10'.
At the same time, the other portion of the laser beam from the aforesaid common laser source is reflected from beam splitter 13 and further reflected by a mirror 18. The reflected beam from mirror 18 is passed through an auxiliary lens 19 where the beam has its width contracted and which is then reflected by a mirror 20. This latter reflected beam from mirror 20 is passed through light transmitting portion 12a of mask 12 to form a diverging beam 21 (a spherical wave beam). This diverging beam 21 has a cross-sectional shape and a diverging angle corresponding to those of the laser beam emitted from the semiconductor laser source to be used. The beam 21 is irradiated on record medium 10' as a diverging record beam with its optical axis perpendicular to record medium 10'.
Thus, an interference pattern between beams 17 and 21 is recorded on record medium 10' with both beams having optical axes perpendicular to record medium 10', that is, being in an inline relation relative to each other. The record medium 10' is then developed to obtain hologram lens 10 having the functions of the optical system in FIG. 2, that is, of collimator lens 2, cylindrical lens 3, and objective lens 6. In this case, the distance between record medium 10' and hologram lens 11 is selected so as to have a maximum overlapping portion of beams 17 and 21 on record medium 10'.
The mask 12, however, may be a hindrance to reproduction of reference wave beam 16. However, if mask 12 is made very small in comparison with the area of hologram lens 11 or is attached to the rear surface of hologram lens 11 (the surface thereof in opposition to record medium 10'), any influence of mask 12 in the formation of subject wave beam 17 will be reduced.
The off-axis hologram lens 11 serving as the objective lens in the embodiment of FIG. 4 is produced in the following manner. As shown in FIG. 6, a record reference wave beam 23 (a plane wave beam or similar spherical wave beam) is irradiated on a hologram record medium 11' at an incident angle of 45.degree. from the common laser source (not shown), which is desirably the same source as previously mentioned in regard to FIG. 4. At the same time, the laser beam from the same laser source falls on an objective lens 24 (similar to that of a microscope which focuses it at a point Q and a diverging record subject wave beam 25 (a spherical wave beam) thereafter is irradiated onto record medium 11' with its optical axis perpendicular thereto so as to form an interference pattern with reference wave beam 23. Thus, processed record medium 11' is developed and off-axis hologram lens 11 is finally obtained therefrom.
In addition to the above embodiment using off-axis hologram lens 11 of FIG. 4 as the objective lens, an embodiment using an inline hologram lens will now be described with reference to FIG. 7. A laser beam from the common laser source (similar to that of FIG. 4, although not shown) is partially passed through a beam splitter 27 to form a reproduction reference wave beam 28 (a plane wave beam or similar spherical wave beam), which falls vertically on an inline hologram lens 11 to produce a reproduction subject wave beam 17 (a spherical wave beam) which is focused at the point P. This subject wave beam 17 is irradiated onto hologram record medium 10' as a focused record beam. Meanwhile, the other part of the laser beam from the same laser source is reflected by mirror 18, contracted in width at auxiliary lens 19, reflected again at beam splitter 27, and then passed through light transmitting portion 12a of mask 12 to obtain the diverging record beam 21 (spherical wave beam). This diverging beam 21 is irradiated onto record medium 10' with its optical axis coincident with a normal of record medium 10', that is, perpendicular to record medium 10'. Thus, an interference pattern between beams 17 and 21 is formed on record medium 10' and record medium 10' is developed to obtain hologram lens 10, as previously discussed. In this case, the hologram lens 11 has a low diffraction efficiency and interference stripes at its center are rough so that it is difficult to produce Bragg diffraction. As a result, it is unnecessary to further consider the influence of mask 12 other than in the embodiment of FIG. 4.
The production of inline hologram lens 11 as the objective lens of FIG. 7 will now be discussed in regard to FIG. 8. In particular, an off-axis hologram lens 30 is provided as an objective lens. The laser beam from the laser source (similar to that of FIG. 4, although not shown) is partially passed through the hologram lens 30 and is irradiated vertically onto a hologram record medium 11' as a reproduction reference wave beam 31 (a plane wave beam or similar spherical wave beam). The other part of the laser beam from the same laser source is rendered to fall on hologram lens 30 at an incident angle of 45.degree. as a reproduction reference wave beam 33 (a plane wave beam or similar spherical wave beam) to derive therefrom a diverging reproduction subject wave beam 32 (a spherical wave beam) which diverges after being focused at the point Q. This subject wave beam 32 is irradiated onto record medium 11' with its optical axis perpendicular thereto. Thus, an interference pattern between beams 31 and 32 is formed on record medium 11' which is then developed to obtain inline holograam lens 11.
The optical reproducing heads above described with reference to FIG. 3 through FIG. 8 have various advantages, for example, they are of a small size, are lightweight, use only a few components, are easy to produce and adjust, produce very little warp with the passage of time, and are inexpensive. In addition, when the tracking servo or focus servo operations are carried out, the driving means therefor can also be made in a relatively small size, the power consumption therefor can be lowered, and the upper limit of the frequency range in the vibration mode can be improved. However, the hologram lens serving as the objective lens must be produced before the aiming hologram lens is produced, so that the method of production is troublesome.